[python] Re: Svar: Re: children's python

  • From: Thomas Jahn <tpj@xxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Fri, 17 Oct 2008 11:22:02 +0200

Hej Dirk,
thanks a lot for you calculations and the interpretations! Considering that the pivot angle is okay, there would be not much to play with the trail at the moment because of his short legs. This would only have been possible by choosing even smaller wheels I guess. But I wanted to avoid that. The 305 mm front wheel just fits between pivot and crank at the moment.

Luka is doing very well on the bike. We did not practise as often as we planned, but after about 6 exercises of 30 min to 2 hours max where he mainly tried to get into move he managed a few meters:
http://www.youtube.com/watch?v=pVNthQhZj_A

And already two times after:
http://www.youtube.com/watch?v=E1j0dllH5B4&feature=related

Once he understood the principle (which was really a 'head thing') he became better and better. His learning curve is definitely more steep than mine. It is sooo great to see him enjoying the python. We both love it!

cheers
Thomas

Den 17/10/2008 kl. 10.08 skrev dirk.bonne@xxxxxxx:

Hi Thomas,

I finally got off my lazy ass and made the graph I promised.

I compared it to the baby python, monty and the P4 of Jürgen. The graph depicts the rise of the seat versus turning angle. As you can see it is
the smallest on Lucas bike. The monty is similar to my bike on purpose
(me and Olaf decided it that way). P4 is in a totally different level
because of the "extreme" trail.

Compared to monty the self centering is smaller because of the lesser
trail (monty has -170mm, luca -114mm). I tried setting the trail to -170 for Luca's bike and the self centering got considerable bigger then that
of the monty or my bike. This is due to the larger wheelbase. If you
would shorten the wheelbase the self centering would go down more.

If you want to have more self centering you need to change the trail.
The wheelbase cannot sensibly be lengthened and the second graph shows
that the pivot angle is already at its best. Another idea would be to
use a spring (silent block) as like on a flevo.

How is Luca doing on the bike?

Sorry about the long delay. Anyways, the mailing list here has been very
very quiet, so everybody seems to have had a time out.

Dirk

Thomas Jahn wrote:
Hi Dirk,
here are the data for Luka's python:
Wheel size 305 (in front with a Schwalbe 47 mm).
pivot angle at 60°
Trail 114 mm
seat height 235 mm
Wheel base 1,04 m

cheers
Thomas


Den 04/08/2008 kl. 13.48 skrev Thomas Jahn:

Hej Dirk,
I am just back from vacation. Luka and me went by bike to Jottlands
westcoast. He on the Brompton and me on the trike. Next time on the
pythons. He did his first experience on the python yesterday. And
although it looks rather challenging, I can already see some first
progress. And he did not give up. Great!

Den 15/07/2008 kl. 9.51 skrev dirk.bonne@xxxxxxx:

Thomas Paul Jahn wrote:
Hej Dirk,

Luka's first trial reminded a lot of his first time on an upright.
He was very eager but it will take some time!!

The bike is a little too big still and so is the seat. He says, the seat is hard. The latter can be solved. The former will solve itself.
The front part is rather heavy. With a Sram 5 speed hub. Am I
surprised?!
And the pivot made of brass bushes is pretty rigid. With his low
weight, the self centering is minimal.
Also the long rear part probably reduces the self centering. I
wonder if I simply cut it shorter.

On my bike the self centering is also minimal - just enough to keep
the front piece upright when sitting on it. I don't feel this is a
problem at all, on the contrary, I like it because it allows for
quick & sharp turns. Maybe Luka just needs the chance to learn it
first a bit?

The weight of the front part should be as low as possible. Why
should the sram hub have something to do with that? To me the bike
looks already quite light - the tube at the front could be
shortened, but then you would need to weld it back as Luca gets older.

You mean, the hub does not have any impact, because it is part of the
rotating mass!?? I see.

You could shorten the rear, yes. It looks anyway disproportional
long compared to the rest of the bike. But I think the self
centering will get smaller. Do you have some data about the bike, I
can calculate a comparisation to my bike. I need:
* wheel size
* pivot  angle
* trail
* seat height
* wheel base

I will give you some data about the bike's geometry soon.....

And at the axis of the rear swing, or at the back wheel, I probably
fix some supporting wheels to each side. So I don't have to walk
bended down next to the bike, while Luka is doing his first exercises.


And the seat lean should be less inclined for him to reach the
ground with his hands. I just hesitated reducing the inclination,
as the beginner is not relaxed enough to go very recumbent. I seems
now it would be better anyway...

What do you think?

May be better to recline the seat (and not add support wheels), so
he can touch the ground so he kind find his own balance.

Luka and me agree!!

About the seat: It was extremely easy to make and is very rigid and
stable. After assembling, I glued it with expoxy. I will definetly
make one for my python as well, where I still lack the proper seat
solution. The wooden seat is very easy to attach too. Because its
construction can be adapted to the bike structure. It seems the
idea for the seat brought me a large step further, while doing the
final work on the bike. I strongly recommend it:
http://hebike.borec.cz/sedlo.htm

Very interesting solution and very light!!! Don't understand the
language but the scale I can read: 616gr. How well does the seat fit
(lumber support)?

I am not an orthopaedist, I made the seat a little too big. But Luka
is in a potentuially fast growing age. Now he just measures 126 cm.
But soon.....

Thomas

dirk

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<luca.jpg><luca2.jpg>

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